SMS not sending

I've had my 8x for not much longer than a month. It is an AT&T 8x running on straight talk. For the first month, the phone was working fine. Ever since I went onto my second month though, nothing but calls will work. Texts will come in just fine, but I can't send any, always coming up with the message 'can't send.' I'm not really sure what to do from here on, and have been searching it for a while. Thanks